This invention relates to voltage reference circuits. More particularly, this invention relates to circuits and methods for reducing the quiescent current in voltage reference circuits.
Many different types of circuits require a voltage reference that is relatively independent of temperature fluctuations. For example, in a switching voltage regulator circuit, the circuit""s output voltage is compared with a voltage reference. If the output voltage drops below the voltage reference, the output voltage is increased. Generally, to obtain an output voltage that is largely independent of temperature, the voltage reference must be similarly independent of temperature.
Voltage references must also be unaffected by variations in circuit element characteristics. Even circuit elements in integrated circuits made from the same manufacturing process will vary at least slightly from one another. If a voltage reference is dependent on characteristics of circuit elements that vary from chip to chip, the corresponding voltage references produced by the integrated circuits will not be identical. Circuit designers face the challenge of ensuring that voltage references are not a function of such characteristics.
Circuit designers also face the challenge of decreasing the power consumption of voltage reference circuits. Again, a voltage regulator circuit serves as a good example. It is highly desirable to decrease the power consumption of voltage regulators that are employed in portable electronic battery-operated devices such as cell phones. These devices tend to experience short periods of high power use (i.e., periods during which relatively large currents must be supplied to a load), followed by extended periods of low power use (i.e., xe2x80x9cstandbyxe2x80x9d time during which a very small load current flows, but a regulated output voltage must be maintained). If the standby periods far exceed the usage periods, the quiescent current (i.e., the input current that flows into the voltage regulator when the output is unloaded but still in voltage regulation) will determine the effective life of the battery. Accordingly, it is desirable to reduce quiescent current consumption as much as possible to extend battery life.
It is difficult to reduce the quiescent current consumption in prior art voltage reference circuits without affecting their accuracy. FIG. 1 shows a typical prior art voltage reference circuit 10, known as a bandgap reference circuit, whose quiescent current is difficult to reduce without affecting the constancy of the voltage reference produced by the circuit. As shown, the voltage reference circuit 10 comprises an op-amp 12, resistors 14, 16 and 18, and a first and second plurality of diodes 20 and 22. The resistors 14 and 18 are coupled between the output of the op-amp 12 and the non-inverting and inverting terminals of the op-amp 12, respectively. The output of the op-amp 12 constitutes the voltage produced by the voltage reference circuit 10.
The voltage reference circuit 10 draws current through the resistor/diode paths and also through the op-amp 12. Typical values for resistors 14, 16 and 18 are 72.6 kohm, 10 kohm and 72.6 kohm, respectively. These resistor values, along with standard diode voltage drops, result in a voltage reference(Vref) of 1.25 V. Given a typical op-amp used in voltage regulator circuits, the total current consumed by the circuit is 26.6 xcexca, 16.6 xcexca of which pass through the resistor/diode paths. To decrease this current consumption, the values of resistors 14, 16 and 18 may be increased. However, if these resistors are too large, leakage currents cause the value of Vref to vary unacceptably.
In view of the foregoing, it would be desirable to provide a circuit and method for operating voltage reference circuits at very low quiescent current levels during standby periods. It is also desirable that such voltage circuits produce a voltage that is relatively independent of temperature and process fluctuations.
More generally, it would be desirable to provide a circuit and method for decreasing the current consumption of voltage reference circuits.
It is therefore an object of the present invention to provide a circuit and method for decreasing the current consumption of voltage reference circuits. It as another object of the present invention to provide a circuit and method for providing a voltage reference that maintains a relatively constant output voltage over a wide range of temperatures and differences in circuit element characteristics that result from process variations.
In accordance with these and other objects of the present invention, a voltage reference circuit capable of operating at reduced quiescent currents is described. The voltage reference circuit comprises an output circuit, a timer circuit and a control circuit. When in standby mode, in order to decrease power consumed by the output circuit, current through the output circuit is decreased, allowing the voltage at an output terminal to fall outside of a desired range. The desired range will depend on design goals that are specific for each circuit that needs a voltage reference. Generally, the desired range will be smaller for circuits that require greater levels of accuracy of a voltage reference.
The control circuit includes a test circuit that generates a test signal characterized by a voltage that emulates changes occurring in the voltage at the output terminal. Specifically, changes in the test signal voltage occur without feedback from the voltage at the output terminal. (This does not imply that the test circuit can not receive feedback from the voltage at the output terminal.) The test circuit comprises circuit elements that correspond to circuit elements in the output circuit, thereby enabling the test circuit to independently generate a voltage that is related to the voltage produced by the output circuit. The test signal voltage changes more rapidly than the voltage at the output terminal, which mitigates the effect of temperature changes on the operation of the voltage reference circuit, as will become apparent from the detailed description.
When the test signal indicates the voltage at the output terminal has fallen outside of the desired range, the control circuit generates a control signal. In response to the assertion of the control signal, the output circuit increases its current draw, thereby changing the voltage at the output terminal such that it falls within the desired range. More particularly, the control signal activates the timer circuit such that the timer circuit generates a timing signal. The output circuit increases its current draw for an amount of time determined by the timing signal, thereby allowing the voltage at the output node to reach the desired range. When this occurs, the current through the output circuit is once again decreased.